The rats we have are Rattus rattus, which climb trees and run along wires. I've seen them run up a stucco house wall and they can dive under a closed door without breaking stride. All they need is half an inch and they're in. All my ventilation places are covered with quarter inch hardware cloth. (this is actually heavy wire mesh, despite the name.) To kill them, I tie a piece of thick string on the bait ledge of a breakneck rat trap. I smear peanut butter on the string. Then I set the trap in a tunnel constructed from bricks or a box made of pieces of lumber. The important thing is to make sure a dog or cat isn't going to get at the trap. I leave the trap unset until the rats get used to eating from it, and then set it just before dusk which is when rats emerge to feed. Diane Whitehead Victoria, B.C.